3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) cellular communications networks utilize Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform (DFT)-spread OFDM in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. As illustrated in FIG. 2, in the time domain, LTE downlink transmissions are organized into radio frames of 10 milliseconds (ms), where each radio frame consists of ten equally-sized subframes of length Tsubframe=1 ms. Each subframe includes two slots of 0.5 ms. Furthermore, resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled. More specifically, in each subframe of the downlink from a base station, the base station transmits control information about to which terminals data is transmitted and upon which resource blocks the data is transmitted in the subframe. This control signaling is typically transmitted in the first 1, 2, 3, or 4 OFDM symbols in each subframe of the downlink. A downlink subframe with three OFDM symbols as control is illustrated in FIG. 3.
LTE utilizes a Hybrid Automatic Repeat Request (HARQ) scheme in the downlink. More specifically, a base station transmits data to a terminal in a subframe of the downlink. At the terminal, after receiving data in the subframe, the terminal attempts to decode the data and then reports to the base station whether the decoding was successful (ACK) or unsuccessful (NAK). In case of an unsuccessful decoding attempt, the base station can retransmit the data.
In LTE, uplink control signaling from the terminal to the base station consists of: HARQ acknowledgements for received downlink data; channel status reports to report downlink channel conditions, which are used for downlink scheduling; and uplink scheduling requests indicating that the terminal needs uplink resources for uplink data transmissions. If the terminal has not been assigned an uplink resource for data transmission, L1/L2 control information (i.e., channel status reports, HARQ acknowledgments (ACKs/NAKs), and uplink scheduling requests) is transmitted in uplink resources (resource blocks) specifically assigned for uplink L1/L2 control on the Physical Uplink Control Channel (PUCCH).
As illustrated in FIG. 4, the uplink resources allocated to L1/L2 control information on the PUCCH are located at the edges of the total available cell bandwidth. Each of these uplink resources consists of 12 subcarriers (one resource block) within each of the two slots of an uplink subframe. In order to provide frequency diversity, these frequency resources are frequency hopping on the slot boundary, i.e., one uplink resource consists of 12 subcarriers at the upper part of the spectrum within the first slot of a subframe and an equally sized resource at the lower part of the spectrum during the second slot of the subframe or vice versa. If more uplink resources are needed for the uplink L1/L2 control signaling, e.g., in case of very large overall transmission bandwidth supporting a large number of users, additional uplink resources blocks can be assigned next to the previously assigned uplink resource blocks. The reasons for locating the PUCCH resources at the edges of the overall available spectrum are two-fold, namely: (1) together with the frequency hopping described above, locating the uplink resources for the PUCCH at the edges of the overall available spectrum maximizes the frequency diversity experienced by the L1/L2 control signaling and (2) assigning the uplink resources for the PUCCH at other positions within the spectrum, i.e., not at the edges, would fragment the uplink spectrum, making it impossible to assign very wide transmission bandwidths to a single mobile terminal and still retain the single-carrier property of the uplink transmission.
The bandwidth of one PUCCH resource block during one subframe is too large for the control signaling needs of a single terminal. Therefore, to efficiently exploit the PUCCH resources set aside for control signaling, PUCCH transmissions from multiple terminals can share the same PUCCH resource block. More specifically, PUCCH transmissions from multiple terminals can be multiplexed onto the same PUCCH resource block by assigning the terminals different orthogonal phase rotations of a cell-specific length-12 frequency-domain sequence, which is referred to as a base sequence. A linear phase rotation in the frequency domain is equivalent to applying a cyclic shift in the time domain. Thus, although the term “phase rotation” is sometimes used herein, the term “cyclic shift” is also used with an implicit reference to the time domain.
The resource used by a PUCCH transmission is therefore not only specified in the time-frequency domain by the resource-block pair, but also by the phase rotation applied to the base sequence. Similarly to the case of reference signals, there are up to 12 different phase rotations specified, providing up to 12 different orthogonal sequences from each cell-specific base sequence. However, in the case of frequency-selective channels, not all the 12 phase rotations can be used if orthogonality is to be retained. Typically, up to six phase rotations are considered usable in a cell.
As mentioned above, uplink L1/L2 control signaling includes HARQ acknowledgements, channel status reports, and uplink scheduling requests. These different types of messages and combinations thereof are transmitted using different PUCCH formats. There are generally two different PUCCH formats defined for LTE. The first PUCCH format is PUCCH format 1. In general, PUCCH format 1 can be used to transmit a HARQ acknowledgement (ACK/NACK) and/or an uplink scheduling request. A HARQ acknowledgement is used to acknowledge the reception of one (or two in case of spatial multiplexing) transport blocks in the downlink. An uplink scheduling request is used to request resources for uplink data transmission. Obviously, a scheduling request should only be transmitted when the terminal is requesting resources, otherwise the terminal should be silent in order to save battery resources and not create unnecessary interference. Hence, unlike a HARQ acknowledgement, no explicit information bit is transmitted by a scheduling request. Rather, the scheduling request is instead conveyed by the presence (or absence) of energy on the corresponding PUCCH resource. However, uplink scheduling requests, although used for a completely different purpose, share the same PUCCH format as HARQ acknowledgements. This PUCCH format is referred to as PUCCH format 1 in the LTE specifications.
As illustrated in FIG. 5, PUCCH format 1 uses the same structure in the two slots of a subframe. For transmission of a HARQ acknowledgement, the single HARQ acknowledgement bit is used to generate a Binary Phase Shift Keying (BPSK) symbol (in case of downlink spatial multiplexing the two acknowledgement bits are used to generate a Quadrature Phase Shift Keying (QPSK) symbol). On the other hand, for a scheduling request, the BPSK/QPSK symbol is replaced by a constellation node treated as a negative acknowledgement (NACK) at the base station. The modulation symbol is then used in combination with a length-12 phase-rotated sequence (i.e., a phase-rotated base sequence) and an Orthogonal Cover Code (OCC) (i.e., the length-4 and length-3 sequences) to generate the signal to be transmitted in each of the two PUCCH slots.
A resource used for a PUCCH format 1 transmission for either a HARQ acknowledgement or a scheduling request is represented by a single scalar resource index. From the resource index, the phase rotation for the base sequence and the OCC are derived. For a HARQ acknowledgement, the resource index to use for transmission of the HARQ acknowledgement is given implicitly by the downlink control signaling used to schedule the corresponding downlink transmission to the terminal. Thus, the resources used for HARQ acknowledgements vary dynamically and depend on the Physical Downlink Control Channel (PDCCH) used to schedule the terminal in each subframe. In addition to dynamic scheduling HARQ acknowledgements based on the PDCCH, HARQ acknowledgements may be semi-persistently scheduled. More specifically, downlink transmissions to a terminal may be semi-scheduled using a semi-persistent scheduling pattern. In this case, configuration of the semi-persistent scheduling pattern includes information on the PUCCH index to use for the corresponding HARQ acknowledgements. In a similar manner, configuration information informs the terminal of the PUCCH resource to use for transmission of scheduling requests.
Thus, to summarize, PUCCH format 1 resources are split into two parts, namely, a semi-static part and a dynamic part. The semi-static part is used for uplink scheduling requests and HARQ acknowledgements for semi-persistently scheduled downlink transmissions. The amount of resources used for the semi-static part of PUCCH format 1 resources does not vary dynamically. The dynamic part is used for dynamically scheduled terminals. As the number of dynamically scheduled terminals varies, the amount of resources used for the dynamic PUCCH format 1 resources varies.
In addition to PUCCH format 1, a second PUCCH format, PUCCH format 2, is defined for LTE. PUCCH format 2 is used to transmit channel status reports. Channel status reports are generated by the terminal and provided to the base station in order to provide an estimate of downlink channel properties at the terminal. The channel status reports are utilized by the base station to aid in channel-dependent scheduling. A channel status report consists of multiple bits per subframe. PUCCH format 1, which is capable of at most two bits of information per subframe, can obviously not be used for this purpose. As such, transmission of channel status reports on the PUCCH is instead handled by PUCCH format 2, which is capable of multiple information bits per subframe.
PUCCH format 2 is illustrated in FIG. 6 for normal cyclic prefix. Like PUCCH format 1, PUCCH format 2 is based on a phase rotation of the same cell-specific base sequence as used for PUCCH format 1. Similarly to PUCCH format 1, a resource used for a PUCCH format 2 transmission can be represented by a resource index. The phase rotation and other parameters for PUCCH format 2 can then be derived from the resource index. The PUCCH format 2 resources are semi-statically configured.
The signals described above for both PUCCH format 1 and PUCCH format 2 are, as already explained, transmitted on a resource-block pair with one resource block in each slot. The resource-block pair to use for a particular PUCCH transmission is determined from the PUCCH resource index. Thus, the resource-block number to use in the first and second slot of a subframe can be expressed as:RBnumber(i)=f(PUCCH index,i)where i is the slot number (0 or 1) within the subframe and f is a function found in the LTE specification.
Multiple resource-block pairs can be used to increase the control-signaling capacity. When one resource-block pair is full, the next PUCCH resource index is mapped to the next resource-block pair in sequence. The mapping is in principle done such that PUCCH format 2 (channel status reports) is transmitted closest to the edges of the uplink cell bandwidth with the semi-static part of PUCCH format 1 next and finally the dynamic part of PUCCH format 1 in the innermost part of the bandwidth. Three semi-statically parameters are used to determine the resources to use for the different PUCCH formats, namely: (1) NRB(2), which is provided as part of the system information, controls the resource-block pair on which the mapping of PUCCH format 1 begins, (2) NPUCCH(1) controls the split between the semi-static and the dynamic part of PUCCH format 1, and (3) X controls the mix of PUCCH format 1 and PUCCH format 2 in one resource block. In most cases, the configuration is done such that the two PUCCH formats are mapped to separate sets of resource blocks, but there is also a possibility to have the border between PUCCH format 1 and PUCCH format 2 within a single resource block. The PUCCH resource allocation in terms of resource blocks are illustrated in FIG. 7. The numbers 0, 1, 2, . . . represent the order in which the resource blocks are allocated to PUCCH, i.e., a large PUCCH configuration may need resources 0-6 while a small configuration may use only 0.
Thus far, the discussion has focused on PUCCH for a single carrier bandwidth. Starting in LTE Rel-10, LTE supports bandwidths larger than 20 megahertz (MHz). In order to assure backward compatibility with LTE Rel-8 which supports a single bandwidth up to 20 MHz, starting with LTE Rel-10, a carrier having a bandwidth that is larger than 20 MHz appears as a number of LTE carriers to an LTE Rel-8 terminal using Carrier Aggregation (CA). CA is illustrated in FIG. 8. Each carrier in an aggregated bandwidth is referred to as a Component Carrier (CC). CA implies that an LTE Rel-10 or later terminal can receive multiple CCs, where the CCs have, or at least have the possibility to have, the same structure as an LTE Rel-8 carrier. The number of aggregated CCs as well as the bandwidth of the individual CCs may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CCs in downlink and uplink is the same whereas an asymmetric configuration refers to the case where the number of CCs is different. It is important to note that the number of CCs configured in the network may be different from the number of CCs seen by a terminal. A terminal may, for example, support more downlink CCs than uplink CCs, even though the network offers the same number of uplink and downlink CCs.
During initial access, an LTE Rel-10 terminal behaves similar to an LTE Rel-8 terminal. Upon successful connection to the network, a terminal may, depending on its own capabilities and the network, be configured with additional CCs in the uplink and the downlink. Configuration is based on Radio Resource Control (RRC) signaling. Due to the heavy signaling and the rather slow speed of RRC signaling, a terminal may be configured with multiple CCs even though not all of them are currently used. A terminal configured on multiple CCs implies the terminal has to monitor all downlink CCs for the PDCCH and Physical Downlink Shared Channel (PDSCH). This implies a wider receiver bandwidth, higher sampling rates, etc. resulting in high power consumption.
To mitigate the above problems, LTE Rel-10 supports activation of CCs in addition to configuration of CCs. The terminal monitors only configured and activated CCs for PDCCH and PDSCH. Alternatively, LTE Rel-11 terminals may monitor an enhanced PDCCH (ePDCCH), which is only detectable by LTE Rel-11 terminals and beyond. Since activation is based on Media Access Control (MAC) elements, which are faster than RRC signaling, activation/deactivation can follow the number of CCs that is required to fulfill the current data rate needs. Upon arrival of large data amounts, multiple CCs are activated, used for data transmission, and deactivated if no longer needed. All but one CC, the DL Primary CC (DL PCC), can be deactivated. Therefore, activation provides the possibility to configure multiple CCs but only activate them on an as needed basis. Most of the time, a terminal has one or very few CCs activated, resulting in a lower reception bandwidth and thus battery consumption.
Scheduling of a CC is done on the PDCCH or ePDCCH via downlink assignments. Control information on the PDCCH or ePDCCH is formatted as a Downlink Control Information (DCI) message. In LTE Rel-8, a terminal only operates with one downlink and one uplink CC, and the association between downlink assignment, uplink grants, and the corresponding downlink and uplink CCs is therefore clear. However, in LTE Rel-10, two modes of carrier aggregation need to be distinguished. The first mode is very similar to the operation of multiple LTE Rel-8 terminals where a downlink assignment or uplink grant contained in a DCI message transmitted on a CC is either valid for the downlink CC itself or for associated (either via cell-specific or terminal specific linking) uplink CC. The second mode of operation augments a DCI message with a Carrier Indicator Field (CIF). A DCI containing a downlink assignment with a CIF is valid for that downlink CC indicted with the CIF and a DCI containing an uplink grant with a CIF is valid for the indicated uplink CC.
DCI messages for downlink assignments contain, among other things, resource block assignments, modulation and coding scheme related parameters, HARQ redundancy version, etc. denoted as DCI format Information Elements (IEs). In addition to those parameters that relate to the actual downlink transmission, most DCI formats for downlink assignments also contain an IE which is a bit field for Transmit Power Control (TPC) commands. These TPC commands are used to control the uplink power control behavior of the corresponding PUCCH that is used to transmit the HARQ feedback.
PUCCH transmission with carrier aggregation is performed somewhat differently that PUCCH transmission for a single carrier. From a terminal perspective, both symmetric and asymmetric uplink/downlink CC configurations are supported. For some of the configurations, one may consider the possibility to transmit the uplink control information on multiple PUCCH or multiple uplink CCs. However, this option is likely to result in higher power consumption at the terminal and a dependency on specific terminal capabilities. It may also create implementation issues due to inter-modulation products, and would lead to generally higher complexity for implementation and testing. Therefore, the transmission of PUCCH should have no dependency on the uplink/downlink CC configuration, i.e., as a design principle all uplink control information for a terminal is transmitted on a single uplink CC. In the case of PUCCH, the single uplink CC used for CA PUCCH is the semi-statically configured and terminal specific Uplink Primary CC (UL PCC), which is referred to as an anchor carrier.
Terminals only configured with a single downlink CC (which is then the DL PCC) and a single uplink CC (which is then the UL PCC) transmit HARQ acknowledgements on dynamically scheduled PUCCH resources according to LTE Rel-8. In this case, the first Control Channel Element (CCE) used to transmit PDCCH for the downlink assignment determines the dynamic PUCCH resource for a corresponding HARQ acknowledgement. If only one downlink CC is cell-specifically linked with the UL PCC, no PUCCH collisions can occur since all PDCCHs are transmitted using different first CCEs.
In a cell asymmetric carrier aggregation scenario or for other reasons, multiple downlink CCs may be cell-specifically linked with the same uplink CC. Terminals configured with same uplink CC but with different downlink CCs (with any of the downlink CCs that are cell-specifically linked with the uplink CC) share the same UL PCC but have different DL PCCs. Terminals receiving their downlink assignments from different downlink CCs will transmit their HARQ feedback on the same uplink CC. It is up to base station scheduling to ensure that no PUCCH collisions occur. However, at least in LTE Rel-10, a terminal cannot be configured with more uplink CCs than downlink CCs.
This concept can be extended even to terminals which have multiple downlink CCs configured. Each PDCCH or ePDCCH transmitted on the DL PCC has, according to LTE Rel-8, a PUCCH resource reserved on the UL PCC. Even though a terminal is configured with multiple downlink CCs but only receives a DL PCC assignment it should still use the LTE Rel-8 PUCCH resource on the UL PCC. The alternative would be to also use, for a single DL PCC assignment, a PUCCH format that enables feedback of HARQ bits corresponding to the number of configured CCs (which is referred to herein as CA PUCCH). Since configuration is a rather slow process and a terminal may be often configured with multiple CCs even though only the DL PCC is active and used, this would lead to a very inefficient usage of CA PUCCH resources.
Upon reception of downlink assignments on a single Secondary CC (SCC) or reception of multiple downlink assignments, CA PUCCH should be used. While in the latter case it is obvious to use CA PUCCH because only CA PUCCH supports feedback of HARQ bits of multiple CCs, it is less clear to also use CA PUCCH in the first case. A downlink SCC assignment alone is not typical. The base station scheduler should strive to schedule a single downlink CC assignment on the DL PCC and try to deactivate all SCCs if not needed. Another issue is that PDCCH for a downlink SCC assignment is transmitted on the SCC (assuming CIF is not configured) and, hence, there is no automatically reserved LTE Rel-8 PUCCH resource on the uplink PCC. Using the LTE Rel-8 PUCCH for stand-alone downlink SCC assignments would require reserving LTE Rel-8 resources on the uplink PCC for any downlink CC that is configured by any terminal using this UL PCC. Since stand-alone SCC assignments are atypical, this would lead to an unnecessary over provisioning of LTE Rel-8 PUCCH resources on the UL PCC.
Also, a possible error case that may occur is that the base station schedules a terminal on multiple downlink CCs including the DL PCC. If the terminal misses all but the DL PCC assignment, the terminal will use LTE Rel-8 PUCCH instead of CA PUCCH. To detect this error case, the base station has to monitor both the LTE Rel-8 PUCCH and the CA PUCCH.
Depending on the number of actually received downlink assignments, the terminal has to provide a corresponding number of HARQ acknowledgements (i.e., a corresponding number of HARQ feedback bits). In a first case, the terminal could adopt the CA PUCCH format according to the number of received assignments and provide feedback accordingly. However, PDCCH with downlink assignments can get lost. Therefore, adopting a CA PUCCH format according to the number of received downlink assignments is ambiguous and would require testing of many different hypotheses at the base station. Alternatively, the CA PUCCH format could be set or included in an activation message. It has been decided that activation/deactivation of CCs is done with MAC control elements and per-CC activation/deactivation is to be supported in LTE Rel-10 and beyond. MAC signaling and especially the HARQ acknowledgement indicating whether the activation command has been received successfully is error prone. Also, this approach requires testing of multiple hypotheses at the base station. Therefore, basing the CA PUCCH format on the number of configured CCs appears to be the safest choice. Configuration of CCs is based on RRC signaling. After successful reception/application of a new CC configuration, a confirmation message is sent back, which makes RRC signaling very safe.
The CA PUCCH can be transmitted in two different ways. The first method is based on the use of PUCCH format 3, which is based on Discrete Fourier Transform Spread (DFTS)-OFDM. PUCCH format 3 is illustrated in FIG. 9. FIG. 10 illustrates the CA PUCCH transmission scheme that is based on the PUCCH format 3, where only one slot is shown. As illustrated in FIG. 10, the multiple HARQ ACK/NACK bits are encoded to form 48 coded bits. The coded bits are then scrambled with cell-specific (and possibly DFTS-OFDM symbol dependent) sequences. Twenty-four (24) bits are transmitted within the first slot and the other 24 bits are transmitted within the second slot. The 24 bits per slot are converted into 12 QPSK symbols, DFT precoded, spread across five DFTS-OFDM symbols and transmitted within one resource block (bandwidth) and five DFTS-OFDM symbols (time). The spreading sequence (OC) is terminal specific and enables multiplexing of up to five users within the same resource blocks. For the reference signals, cyclic shifted Constant Amplitude Zero AutoCorrelation (CAZAC) sequences can be used. Some examples of CAZAC sequences that can be used are the computer optimized sequences in disclosed in 3GPP Technical Specification (TS) 36.211, “Physical Channels and Modulation.”
The second CA PUCCH transmission scheme is referred to as channel selection. The basic principle is that the terminal is assigned a set of PUCCH format 1a/1b resources. The terminal then selects one of resources according to the HARQ ACK/NACK sequence that the terminal should transmit. The terminal then transmits a QPSK or BPSK on the selected resource. The base station detects which resource the terminal uses and which QPSK or BPSK value the terminal fed back on the used resource and combines this into a HARQ acknowledgement for associated downlink CCs. The mapping of ACK (A), NACK (N) and DTX (D) is according to FIG. 11A, FIG. 11B and FIG. 11C for Frequency Division Duplexing (FDD). A similar type of mapping including a bundling approach is also done for Time Division Duplexing (TDD) in case the terminal is configured with channel selection.
The discussion above describes PUCCH for conventional LTE deployments. However, as new deployments (e.g., heterogeneous cellular communications networks) emerge, there is a need for PUCCH transmission schemes for these new deployments.